In transmit diversity systems, one of the advantageous features of a mobile transmit diversity communication device is an extension of operating range. This effect may be an outcome of forming a beam that exhibits a higher Effective Radiated Power (ERP) than a conventional transmitter that does not employ transmit diversity. This advantageous feature may be enhanced by a successful and/or efficient beamforming process, for instance, in a closed loop mechanism, on the provision of feedback from the receiver to the transmit diversity transmitter.
Generally, in systems of communication between a mobile handset, for example, a mobile telephone, personal digital assistant (PDA), laptop computer, etc. and a base station over a random access channel, there is typically a protocol for the mobile device to initiate communication with the base station. Typically, the mobile device will attempt to access a network by emitting an access probe, also known as a preamble signal. If no response is received from the base station, the mobile device may repeat transmission of the access probe with increased power, and repeat increasing power of the probes until an end condition is reached. An end condition may be receiving acknowledgement, or it may be when the maximum allowed probe power is reached, or when the maximum allowed number of probes is sent. If an acknowledgment is not received, the mobile device will typically declare the access attempt a failure and try again, either automatically or manually.
Therefore, in cellular communication systems, the mobile transmitter may begin a registration process to the network, but the base station has not yet registered the existence of the mobile transmitter. In some implementations of transmit diversity, the transmitter may require feedback from the receiver, e.g., a base station, in order to optimally adjust a phase difference or other diversity parameter. However, in the absence of successful registration, the base station may not provide the feedback required to close the loop, as is advantageous for purposes of transmit diversity. This lack of feedback may render inefficient the proper beamforming resultant ERP and reduce the potential range extension. This effect may be further adversely affected by the usage of multiple sub-power power amplifiers, which may possibly be used for saving or reducing current consumption, when beamforming is successfully implemented.
As described in prior patent applications of the assignee of the present invention, a mobile transmit diversity (MTD) device may use two antennas simultaneously transmitting with a transmit diversity parameter, for example, a phase difference or a power ratio between at least two antennas. In one embodiment, two “diversity power amplifiers” may be used to save the current consumption for the mobile device, as described, for example, in US Patent Publication No. 2003/0002594, the entire contents of which are incorporated herein by reference. Use of mobile transmit diversity may permit extending the effective range of a mobile device from a base station using the same power as a non-diverse transmission device.
The maximum power of this diversity power amplifier can be half or even quarter of the maximum power of the “regular” power amplifier. However, when a mobile transmitter accesses the wireless network, it may need the maximum power from the mobile power amplifier to get base station's attention (e.g., at the cell edge). There is therefore a need for an efficient initial negotiation process for a mobile transmit diversity communication device.